In the manufacture of integrated circuits, copper interconnects are generally formed on a semiconductor substrate using a copper dual damascene process. Such a process begins with a trench being etched into a dielectric layer and filled with a barrier layer, an adhesion layer, and a seed layer. A physical vapor deposition (PVD) process, such as a sputtering process, may be used to deposit a tantalum nitride (TaN) barrier layer and a tantalum (Ta) or ruthenium (Ru) adhesion layer (i.e., a TaN/Ta or TaN/Ru stack) into the trench. The TaN barrier layer prevents copper from diffusing into the underlying dielectric layer. The Ta or Ru adhesion layer is required because the subsequently deposited metals do not readily adhere or nucleate on the TaN barrier layer. This may be followed by a PVD sputter process to deposit a copper seed layer into the trench. An electroplating process is then used to fill the trench with copper metal to form the interconnect.
As device dimensions scale down, the aspect ratio of the trench becomes more aggressive as the trench becomes narrower. This gives rise to issues such as trench overhang during the copper seed deposition and plating processes, leading to pinched-off trench openings and inadequate electroplating gapfill. Additionally, as trenches decrease in size, the ratio of barrier metal to copper metal in the overall interconnect structure increases, thereby increasing the electrical line resistance and RC delay of the interconnect.
One approach to addressing these issues is to reduce the thickness of the TaN/Ta or TaN/Ru stack, which widens the available gap for subsequent metallization and increases the final copper volume fraction. If a physical vapor deposition (PVD) deposition is used to form a thinner barrier layer, the line of sight deposition does not allow good sidewall coverage at aggressive geometries. As a result, the barrier layer occupies a significant amount of volume of the interconnect (e.g., greater than 20%), which increase the electrical resistance of the metal interconnect. Current atomic layer deposition (ALD) barriers are nitrided barriers with some carbon, and pure copper does not adhere or nucleate well to such layers, thereby causing adhesion issues and potential electromigration failures at short stress times. Accordingly, alternative techniques for reducing the thickness of the barrier and adhesion layer are needed.